The older meaning defines a Blue Moon as the third full moon in a season that has four full moons. Called a seasonal Blue Moon, this occurs about every 2.5 years, according to NASA. More recently, the term Blue Moon has been applied to the second full moon within a single calendar month. There are roughly 29.5 days between full moons, making it unusual for two full moons to fit into a 30- or 31-day-long month. (This means that February will never have a Blue Moon.)
Which one is correct? Well, since language and the meaning of words constantly evolve — take your pick. Both are commonly used today and either definition is considered valid. As Texas astronomer Donald W. Olson wrote in a 2006 column for Sky & Telescope magazine, "With two decades of popular usage behind it, the second-full-moon-in-a-month (mis)interpretation is like a genie that can't be forced back into its bottle. But that's not necessarily a bad thing."
The next monthly Blue Moon (a second full moon that appears in a single month) will be on Oct. 31, 2020, according to NASA. The last one was on March 31, 2018. There was a seasonal Blue Moon on Saturday, May 21, 2016, and the next one will be in the spring of 2019. The third full moon of that season will be on May 18, 2019.
Origin of the term
The phrase "once in a Blue Moon" has been around for more than 400 years, according to Philip Hiscock, a folklorist at the Memorial University of Newfoundland. In a 2012 article in Sky & Telescope magazine, he explained that the earliest use of term was much like saying the moon is made of green cheese — it indicated something absurd. "He would argue that the moon is blue" was similar to saying, "He would argue that black is white."
The meaning evolved to something akin to "never," according to Hiscock. "I'll marry you when the moon turns blue" became the equivalent of "I'll marry you when pigs fly."
"Most Blue Moons look pale gray and white, indistinguishable from any other moon you've ever seen," according to NASA. "Squeezing a second full moon into a calendar month doesn't change the physical properties of the moon itself, so the color remains the same."
But, never say never. It turns out that the moon can appear bluish, as it did in 1883 after the volcano Krakatoa erupted. Dust in the air acted as a filter, causing sunsets and the moon to turn green and blue all over the world, an event that NASA said is thought to have spawned the phrase ”blue moon." Other events such as forest fires and dust storms can cause the moon to turn blue.
So, the meaning of "once in a blue moon" changed from "never" to "rarely," according to Hiscock.
When does a Blue Moon occur?
Now we get to the contradictory scheduling definitions of Blue Moon.
The seasonal definition for the term Blue Moon has been traced back to the August 1937 issue of the Maine Farmer's Almanac, which explained that the moon "usually comes full 12 times in a year, three times for each season. Occasionally, however, there will come a year when there are 13 full moons during a year, not the usual 12. And that extra full moon also meant that one of the four seasons would contain four full moons instead of the usual three."
The almanac followed certain rules for what to call each moon. For example, the last full moon of winter had to fall during Lent; it was called the Lenten Moon. The first full moon of spring was called the Egg Moon — or Easter Moon, or Paschal Moon — and had to fall within the week before Easter. There was also the Moon Before Yule and the Moon After Yule. So when a particular season had four moons, the third was dubbed a Blue Moon, so that the other full moons could occur at the proper times relative to the solstices and equinoxes.
But what about the definition that many people have heard — that a Blue Moon is the second full month in a single month? That came from a 1946 misinterpretation of the original definition, which was used as a source for a nationally syndicated radio program in 1980.
This article was updated on Nov. 15, 2018 by Space.com Senior Writer, Meghan Bartels.